The benzodiazepines probably exert their actions via interaction with specific receptor sites on neurons. Studies during the last year have focused on the characterization of the benzodiazepine receptor using newly available ligands. Studies involving the Beta-carboline-3-carboxylate ethyl ester (Beta-CCE) showed that the binding of both this ligand and the peripheral type receptor ligand, RO-5-4864, are not affected by GABA. Specific peripheral type benzodiazepine receptors were described in brain and characterized. These studies have shown that benzodiazepine agonists and antagonists interact differently with this receptor. The interaction of the methylxanthine stimulant, caffeine, with the benzodiazepine receptor in human brain has been studied; its relative potencies in binding to the adenosine receptor and the benzodiazepine receptor were compared. The finding that caffeine is move potent as an inhibitor of adenosine receptor binding suggests that the adenosine receptor system may mediate some of the anxiogenic effects of this cortical stimulant.